YANGON - Several people including women and a child have been killed in an attack on Rohingya Muslims in strife-torn western Myanmar, a rights group said Friday, as the United States voiced alarm.

Myanmar's Rakhine state remains tense after several outbreaks of communal bloodshed between Buddhist and Muslim communities since 2012 that have killed scores and displaced 140,000 people, mainly from the Rohingya minority.

Details of the latest unrest were unclear, but activists said at least two women and a child were stabbed to death in an attack on a village near the border with Bangladesh earlier this week, with possibly several dozen casualties.

Myanmar authorities denied any civilian deaths but confirmed a clash took place in which a police officer was presumed to have been killed.

Chris Lewa, the Bangkok-based director of The Arakan Project, which lobbies for Rohingya rights, said the attack on the village of Du Chee Yar Tan on Monday happened some time after the initial clash with police.

"There were people killed, mostly women and children," she told AFP, but added that reports from sources in the area on the number of people killed varied widely, from around 10 to several dozen.

US State Department spokeswoman Jen Psaki said Washington was "deeply troubled" by the reports of violence.

"We're particularly disturbed by reports that security forces may have used excessive force in (perpetrating) some of the violence," she added.

Lewa said one villager, who has worked with The Arakan Project, reported seeing the bodies of two women and a 14-year-old boy with stab wounds after returning to the village days after the unrest.

She said the use of knives suggested the involvement of local Rakhine Buddhists, who have repeatedly clashed with the Rohingya, rather than the police.

The Maungdaw area is populated mainly by stateless Rohingya, whose movements are strictly controlled by a heavy security presence.

Aid group Doctors Without Borders (MSF), one of the few outside organisations permitted to operate in the region, said it saw on Wednesday two wounded people "suffering from injuries inflicted as a result of violence", one with a gunshot wound and the other apparently badly beaten.

It said its medical clinic nearby had seen an "unusually low" number of patients Friday, which had caused fears for the local population.

"MSF is concerned that there may be unmet medical needs among the affected population," said Head of Mission Peter Paul de Groote.

Local police denied any villagers had died, but said authorities had come under attack on Monday without giving any reason why.

"A police sergeant is still missing along with his weapon. We are looking for him," a senior police official in nearby Maungdaw town told AFP on condition of anonymity.

Another police officer in the state capital Sittwe said dozens of people had been rounded up after the unrest, with 10 still in custody.

Myanmar's government considers the estimated 800,000 Rohingya in the country to be foreigners while many citizens see them as illegal immigrants from Bangladesh and view them with hostility.

Two rounds of unrest in Rakhine state in June and October 2012, largely between local Buddhists and the Rohingya Muslim minority, sparked religious unrest that has since spread across the country.

About 250 people have been killed in the fighting, which has overshadowed widely praised reforms by a quasi-civilian government that took power in 2011.

Rakhine has been left almost completely segregated on religious and communal grounds by the unrest, with many thousands of Muslims living in squalid camps.